This invention relates to the formation of a conductive layer of metal on a substrate. In particular, this invention relates to such a process wherein the conductive layer is formed from discrete metal particles of copper or nickel, and a chemical developing agent is employed.
It is frequently desired to form a layer of conductive metal on a (generally) non-conductive substrate. Such composites are useful for printed circuit boards, electromagnetic interference (EMI) shielding, and so forth.
Japanese publication JP 110704(84) (shin Gijutsu Kaihat) reports that a conductive layer may be formed on a substrate by spraying super fine (e.g.: 0.2 um) metal particles with the aid of a carrier gas (e.g.: nitrogen or argon) onto a substrate, followed by low temperature (e.g.: 80.degree. C.) sintering. One significant disadvantage of this process is the limited availability of such fine metal dust. Further, the use of metal dust in an air spray system requires extensive hygienic safeguards.
Japanese publication JP 66133(73) (Fujimori Kogyo) and DE 2,163,118 (Fujimori Kogyo (Sakai)) both describe a process where metal particles are mixed with a binder, applied to a substrate, dried, and treated with an agent which is an acid, a halogen, or a halogenide. Such processes are inconvenient and damaging to equipment in their requirement for a binder and a corrosive agent.
GB 2,085,340 teaches a paint containing copper particles and optionally a wetting agent (Preferably triethanolamine), which is useful for producing conductive coatings. It has not teaching of a conductive layer in the absence of a binder.
US 4,434,084 (Dupont (Hicks)) and DE 2,411,988 (Dupont (Beske)) both describe processes wherein a mixture of copper and tin particles are coated on a substrate with the air of chemical activators and heat sufficient to melt the tin. In the U.S. Pat. the chemical activators are an organic acid flux and an organic amine in an inert organic medium. In the German publication the chemical activator is a flux which may contain triethanolamine as an adjuvant. Both patents require the formation of an intermetallic compound (Cu.sub.3 Sn), and neither patent teaches the use of an amine in the absence of another agent.